Not applicable.
Not applicable.
The present invention is generally related to switches for use in micro systems, and more particularly to electronic signals used to change the state of MEMS switches.
Optical switches can be used in a variety of applications, such as optical fiber transmission networks, to route optical signals along various signal paths. An optical switch typically has an optical element, such as a mirror or a filter, that is switched into and out of a path of an optical signal beam. Switches are typically characterized by the number of input and output ports, referred to as Nxc3x97N. For example, a 1xc3x972 switch would switch one input between two outputs.
Switches can often be described as xe2x80x9clatchingxe2x80x9d or xe2x80x9cnon-latchingxe2x80x9d. A latching switch reliably remains in a known position, even if the power is removed or lost. A non-latching switch may revert to an unknown position, or even a position intermediate between switch states, when the power is lost, for example if current provided to an electromagnetic solenoid or thermal actuator is lost. One type of latching switch reverts to a known default position (state), no matter what state the switch was in when power was lost. Another type of latching switch preserves the switch state, no matter what that state was. The latter case is known as a xe2x80x9cbi-stablexe2x80x9d switch.
Bi-stable optical switches are desirable for use in optical telecommunication systems because they preserve the network configuration associated with the position of the switch(es) when the power was lost. Various approaches have been used to produce bi-stable optical switches. One approach uses a permanent magnet in conjunction with a piece of magnetic material to hold the switch element in the desired position. Other approaches use a mechanical latch to hold the switch element in the desired position.
In a particular application, as illustrated and described in U.S. Pat. No. 5,994,816 entitled THERMAL ARCHED BEAM MICROELECTROMECHANICAL DEVICES AND ASSOCIATED FABRICATION METHODS by Dhuler et al., issued Nov. 30, 1999, a mechanical latch is used in a micro-electro-mechanical system (xe2x80x9cMEMSxe2x80x9d) (See, e.g. FIG. 11, ref. nums. 69 and 68c). A thermal arched beam actuator is used to move a switch element back and, with a thermally activated latch holding the switch element in the desired position(s). However, having contact surfaces between the latch and the switch element can result in the mechanism sticking or produces xe2x80x9cstictionxe2x80x9d (i.e. sticking friction), thus altering the force required to change switch states. This sticking or stiction can not only affect the reliability of switch operation, but could also affect the timing of the switch, particularly with fast (i.e. xe2x89xa61 ms) switching in light of the need to time the operation of the latch with the operation of the actuator.
U.S. Pat. No. 5,994,816 also describes a latching mechanism that uses an electrostatic field to clamp a movable portion of the switch to the switch body (substrate). Clamping allows the relatively high current flow to the thermal beam actuator to be removed without losing the clamped switched state, thus conserving power. However, if the voltage to the electrostatic clamping circuit is removed the switch may revert to a state other than what was previously held.
In either latching or non-latching cases, an electronic signal is typically applied to the actuator to change switch states. The electronic signal is generally chosen to have sufficient energy to reliably change switch states within a given period of time. An electronic pulse, typically applied as a square wave pulse, is often used to apply a switching signal with sufficient voltage and duration to switch in a worst-case situation. Variations in required switching energy arise due to fabrication differences between switches, environmental considerations, aging, and other factors to contribute to a worst-case situation. Unfortunately, applying such a switching signal may provide more energy than is needed and may drive the switch to induce ringing or overshoot.
Accordingly, it is desirable to provide an electronic switching signal that reliable switches a MEMS switch with reduced overshoot and ringing.
The present invention provides a method of actuating a MEMS switch with an electronic signal that accelerates a movable portion of the MEMS switch from a first switch position toward a second switch position, and then decelerates the movable portion of the MEMS switch before it reaches the second switch position.
In one embodiment, the switching signal includes a first electronic pulse, a selected dwell period, and a second electronic pulse. The MEMS switch includes an electrostatic comb drive motor as an actuator, for example, having two electrostatic arrays, one array configured to move a movable portion of the switch in a first direction, and the other array configured to move the movable portion of the switch in a second direction in response to applied electrical signals. In one embodiment, a xe2x80x9cpushxe2x80x9d pulse is followed after a selected dwell period with a xe2x80x9cpullxe2x80x9d pulse. In another embodiment, a first push pulse is followed after a longer dwell period by a second push pulse. The pulses may approximate square wave pulses, or may include ramped leading and/or following edges, as well as alternatively or in addition to a sloped pulse top. The combination of serial pulses with the selected dwell period dampens or decelerates the motion of a center body (movable portion) or the switch, which in a particular embodiment is attached to the static portions of the switch with spring arms.
In yet a further embodiment, the MEMS switch requires a first amount of energy to switch from a first state to a second state, and a second (different) amount of energy to switch from the second state back to the first. A MEMS switching cycle includes a first switching signal having a first pulse, a first selected dwell period, and a second pulse; and a second switching signal having a third pulse, a second selected dwell period, and a fourth pulse, wherein the energy available from the first switching signal is less than the energy available from the second switching signal.